White cast irons, and in particular carbon-containing, nickel-chromium bearing iron based alloys such as Ni-Hard.RTM., have long been known in the metallurgical industries for their hardness and ease of castability, and for their relative inexpensiveness. The physical properties of such white cast irons can, within certain limits, be modified by suitable adjustments in the relative ratios of the noted alloying elements. Some further improvements can also be made by additions of other alloying elements, such as for instance copper, molybdenum, tungsten, cobalt. Such additions, however, increase the cost of production of the iron based alloy, and while one or two aspects of its physical properties are extended, some others may be detrimentally affected.
Compositions for nickel and chromium-bearing chill cast irons with good abrasion and oxidation resistance, which can be cast in complex shapes, are described in U.S. Pat. Nos. 1,988,910; 1,988,911 and 1,988,912, and are characterized by the chromium content of these alloys being less than the nickel present. An alloy with similar properties, for thick castings of substantial size, with fine grain structure and good abrasion resistance, is taught in U.S. Pat. No. 2,662,011 with chromium contents less than 15% and having nickel contents between 4 and 8%. The wear and abrasion resistant properties of nickel and chromium bearing white cast irons are described in U.S. Pat. No. 3,410,682 and Canadian Pat. No. 848,900; these alloys contain in addition, manganese and molybdenum in well-defined concentration ranges.
The alloy of U.S. Pat. No. 3,414,442 is specified to have chromium levels below 15% and nickel concentrations between 4 and 8%; in addition this patent also teaches a heat treatment process of the alloy to increase its hardness after casting.
Wear resistant, nickel-bearing white cast irons are described in Russian Pat. No. 583,192 with chromium contents in excess of 20 percent and nickel contents falling between 1.2 and 3.2 percent. The alloy of the Russian patent also contains manganese between 0.4 and 0.6 percent and silicon between 0.6 and 1.0 percent.
The corrosion and erosion resistant white cast iron of U.S. Pat. No. 4,080,198 has a high chromium content, such as in excess of 28%, with molybdenum, nickel and copper additions of less than 2%. According to the heat treatment process taught therein, part of the carbon contained in the alloy as molybdenum and chromium carbides dispersed in the austenitic matrix, can be resolutionized to reduce the hardness of the alloy by a relatively small extent, and the alloy can subsequently be aged back to acquire the desired hardness.
U.S. Pat. Nos. 3,165,400 and 3,235,417 teach oxidation resistant austenitic casting alloy compositions with relatively low carbon contents, having chromium contents between 12 and 35% and nickel contents up to 15%. The alloys with the composition ranges of these two patents, contain several other alloying elements as well, and in addition the nickel, manganese and cobalt concentration levels are interrelated according to a pattern defined therein.
The abrasion resistant nickel, chromium-bearing iron based alloy described by prior art patents hereinabove can be cast in a desired shape. They are, however, not machinable by conventional methods, and any adjustment in size, shape, modification of surface or refinement in critical dimensions, can only be achieved by grinding. Grinding is, as is well known, a costly process, especially on larger pieces, and difficult to control.